Robert George offers an important basis for emphasis here, but there’s an important inward extension to his description of the law:

The law is a teacher. It will teach either that marriage is a reality in which people can choose to participate, but whose contours people cannot make and remake at will, or it will teach that marriage is a mere convention, which is malleable in such a way that individuals, couples, or, indeed, groups can choose to make of it whatever suits their desires, goals, and so on. The result, given the biases of human sexual psychology, will be the development of practices and ideologies that truly tend to undermine the sound understanding and practice of marriage, together with the development of pathologies that tend to reinforce the very practices and ideologies that cause them.

The inward extension is that, as much as the law is a teacher, its “students” in a democratic society must ultimately approve of the lesson. It’s a matter of give and take — mutual reinforcement. The country’s people construct the law, and the law helps to guide their behavior. Regulations act as guidelines toward a desired end.
This same adjustment must be made to an excellent piece by RI locals Michelle Cretella and Arthur Goldberg:

Legislators and justices will do well to heed the findings of J.D. Unwin, British anthropologist and author of Sex and Culture. After studying 86 societies spanning 5,000 years of history he found a distinct correlation between increasing sexual freedom and social decline. Unwin postulated that when social regulations forbid indiscriminate satisfaction of sexual impulses, the sublimated sexual impulses are channeled into a “social energy” that builds society. Conversely, he found no instance in which a society retained its creative energy after abandoning monogamous male-female relationships.

The described findings certainly represent a crucial splash of cold water, but it isn’t merely “legislators and justices” who should feel its chill. Indeed, if such personages come alone to the revelation, it would be inappropriate for them to impose it on an unwilling nation. It is the entire network of intellectual and cultural elites that must heed the warning.
The unique project of the United States is to regulate outside of the law as much as possible. American society comes to agreement about the minimum boundaries within which everybody can achieve their goals, and the law provides those boundaries. The difficulty when it comes to marriage is that one side would like the law to enable its goal of declaring same-sex relationships to be indistinguishable in any profound way from opposite-sex relationships. Cultural elites have proven scandalously blind to the fact that such a proposition is utterly preposterous in just about every light (biology not least among them), requiring traditionalists to point out that the requested modification to the law would make it more difficult for our society to maintain its goal of advancement — even cultural survival.
It would be difficult to overstate the fundamental importance of this debate, because the culture of marriage is perhaps the most significant means of non-government regulation of behavior. Reading Cretella and Goldberg, those of us who trace political threads might find significance in the fact that sexual libertinism is so often married with statist, progressive movements. Just so, it’s difficult not to wonder whether radical redefinition of our entire society isn’t the actual goal of those who wish to modify marriage.